(1) Field of the Invention
The present invention relates to a low shrinkable unsaturated polyester resin composition excellent in the low shrinkable property in cold curing.
(2) Description of the Prior Art
In general, the curing molding of the unsaturated polyester resin is obtained by using an organic peroxide such as benzoylperoxide, methylethyl ketone peroxide or the like as a polymerization catalyst, and if necessary, an organic metal salt such as cobalt naphthenate, cobalt octenate or the like as a polymerization promoter according to the contact pressure molding method such as the hand lay-up method, the cold press method or the resin injection method utilizing the reaction heat generated when the unsaturated polyester resin is cured with the use of a relatively low pressure press or press-in machine, the cold molding method in the case of a resin mortar, resin concrete and the like, or the heating molding method using the composition for molding such as the sheet molding compound (SMC), the bulk molding compound (BMC) and the like.
However, since the cure shrinkage of the unsaturated polyester resin is as much as about 5-12% by volume, the occurrence of various defects such as strength reduction, cracking, warping, surface contamination and the like resulting from the cure shrinkage could not be prevented by adopting any of the above molding methods.
As the method for reducing the above cure shrinkage of the unsaturated polyester resin, there has been employed a method in which the thermoplastic resin such as polystyrene, polymethylmethacrylate, polyvinyl acetate or the like is compounded into the unsaturated polyester resin, and it is widely known that the shrink-resistant effect can be produced to some extent by such a method.
However, there remains unsolved the fundamentally serious defect. That is, in order to make the general thermoplastic resin to exhibit the effect as the shrink-resistant agent to reduce the cure shrinkage of the unsaturated polyester resin, it is always necessary that the molding temperature be considerably high in cure molding. For this reason, there exists no effective shrink-resistant agent for a method other than the heating molding method. Further, the general thermoplastic resin has a poor dispersion stability into the unsaturated polyester resin, so that it is raised from the unsaturated polyester resin molding on the midway of the curing, and the application field is restricted due to roughness of the surface of the cured product, cure insufficiency, ununiformity in the cure shrinkage, strength reduction and so on.
After the present inventors had made strenuous studies to solve the above defects, they found that when copolymer containing mainly of a block copolymer (hereinafter referred to as "a block copolymer mixture") which is composed of vinyl acetate and styrene segment either one of which is bound with an acidic group is added to the unsaturated polyester resin when the unsaturated polyester resin is cured, excellent shrink-resistant effect can be conferred upon the unsaturated polyester resin cured product, and they have filed a patent application therefor under an application No. 48,769/1981 (Japanese Patent Application Laid-Open No. 164,114/1982). Although this method is satisfactory in the molding under heating, it is not satisfactory in the cold curing.